When plastic dispersions are added to hydraulically-setting compositions, such as concrete and mortar compositions, foam is frequently formed on mixing, due to the emulsifier content of the plastic dispersion. The air introduced into the mixture can no longer escape, so that the final strength of the material when set is reduced by the great number of air-filled pores. For this reason anti-foaming agents or defoaming agents are added to the compositions to be mixed. These are mixtures of addition products of ethylene oxide onto alkylphenols, fatty alcohols, fatty amines or fatty acids. These anti-foaming agents are not very effective, even when they are added in large quantities. Other known anti-foaming agents are for example, tertiary esters of o-phosphoric acid, however, on the basis of their chemical constitution, these agents are toxic.
In German publish application, DOS No. 24 03 444, abstracted in Chem. Abstracts 83, 197362.sub.n (1975), there is mentioned as anti-foaming agents, mixtures based on alkoxylated fatty alcohols containing 12 to 22 carbon atoms and alkoxylated products of other substances with these alkoxylated fatty alcohols which are obtained by the addition of propylene oxide onto the fatty alcohols. Dispersions produced with these anti-foaming agents are stable in storage, but the anti-foaming agent loses much of its ability to defoam after mechanical stress, that is, after prolonged mixing periods either of the dispersion or of the unset mortar or concrete.
German published application DOS No. 14 71 100, corresponding to U.S. Pat. No. 3,228,907, describes as an anti-foaming agent a dimethylpolysiloxane/silica/aerogel emulsion, which likewise loses much of its ability to defoam after mechanical stress.